MEMS enclosure

ABSTRACT

A low-cost, high-performance, reliable micromirror package ( 300 ) that replaces the ceramic substrate in conventional packages with a printed circuit board substrate ( 30 ) and a molded plastic case ( 33 ), and the cover glass with a window ( 36 ), preferably an optically clear plastic window. The printed circuit board substrate ( 30 ) allows for either external bond pads or flex cable connection of the micromirror package to the projector&#39;s motherboard. These packages support flexible snap-in, screw-in, ultrasonic plastic welding, or adhesive welding processes to overcome the high cost seam welding process of many conventional packages.

This application claims priority under 35 USC §119(e)(1) of provisionalapplication No. 60/249,105 filed Nov. 16, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the packaging of semiconductor chipsand particularly to that of micro-electrical mechanical systems (MEMS)such as micromirror devices.

2. Description of the Related Art

Packaging is a critical part of producing a high-performance MICROMIRRORfor use in optical applications. Typically in a micromirror, the mirrorsland on the substrate surface of the device. To avoid the mirrors fromsticking, it is necessary that the micromirror be packaged in acontrolled environment, with minimum amounts of moisture, adhesives,dust, and other contaminants. In order to provide such an environment,hermetically sealed packages are often used. However, the cost of suchpackages and the negative impact on the assembly process is extremelyhigh.

The packaging of micromirror chips for use in projection display andother electro-optical applications has continued to present a costbarrier that contributes to higher prices for these products. A lowercost micromirror package is required to reduce the cost of thesehigh-resolution, digital projectors. Today's micromirror packages aremostly built on custom designed ceramic substrates and have expensiveglass covers (lids), which are seam welded or fixed in place with anadhesive. These packages are not only expensive, but they require a lowthroughput process that reduces the product cycle time.

FIG. 1 is a drawing of a welded package. This is a hermetically sealedpackage that consists of a ceramic substrate 10 with a built-in Kovarlid-attaching ring 11 on its surface and a lid assembly, which consistsof a Kovar frame 12 with built-in an optical quality glass window 13. Inthe assembly process the micromirror die 14 is attached to the substrateto provide both mechanical and thermal stability, as well as anelectrical ground plane. The micromirror leads are bonded to pads on thesubstrate 10 which extend to external package pads on the edges orbottom of the package. The package is filled with an inert gas and thenthe lid assembly is seam welded at the two mating surfaces 15 betweenthe lid frame 12 and lid-attaching ring 11.

The adhesive sealed package shown in FIG. 2 has been used in place ofthe welded package to lower the cost and improve the manufacturingthroughput. This package is similar in that it has a ceramic substrate20 but the cover glass 21 (lid) is a single piece of optical qualityglass. In this case, the micromirror 22 is mounted and bonded out in thesubstrate's 20 cavity and then the cover glass 21 is attached using anadhesive. The main advantages are that the assembly process is somewhatsimplified, which improves the manufacturing throughput and the glasscovers 21 are simple pieces of glass that can be sourced from variousvendors.

The micromirror packages discussed above perform very well but are tooexpensive and tend to limit cost reduction efforts due to their highcost material and labor content. What is needed is a simple micromirrorpackaging approach that is low cost, easily assembled, and reliable. Theinvention disclosed herein fulfills this need.

SUMMARY OF THE INVENTION

This invention discloses a low cost, high performance, reliablemicromirror package that overcomes many of the shortcomings of theconventional ceramic packages commonly used. This approach replaces theceramic substrate with a printed circuit board substrate, the ceramiccase with a molded plastic case, and the cover glass with an opticalquality plastic window or lid. The printed circuit board substrateallows for either external bond pads or flex cable connection of themicromirror package to the projector's motherboard. These packagessupport flexible snap-in, screw-in, ultrasonic plastic welding, oradhesive welding processes to overcome the low throughput, high cost,seam welding process of many conventional packages.

Other advantages include the following:

1. Requires no seam welding,

2. readily supports assembly automation,

3. uses multiple source, commodity piece parts,

4. light weight,

5. has built-in thermal plane on the bottom of the printed circuit boardsubstrate,

6. supports standard chip connection methods, such as edge pad,grid-pad, or flex cable, and

7. lower cost.

DESCRIPTION OF THE VIEWS OF THE DRAWINGS

The included drawings are as follows:

FIG. 1 is a perspective view of a conventional welded hermeticallysealed, ceramic, micromirror package. (prior art)

FIG. 2 is a perspective view of a conventional epoxy sealed, ceramic,micromirror package. (prior art)

FIG. 3a is a perspective view of the low-cost, snap-on window,micromirror package of a first embodiment of this invention.

FIG. 3b is a sectional view of one portion of the package of FIG. 3ashowing the lid retention mechanism.

FIG. 4 is an exploded view of the micromirror package in FIG. 3a.

FIG. 5a is a perspective view of the low-cost, adhesive attached window,micromirror package of another embodiment of this invention.

FIG. 5b is a sectional side view of a portion of the package of FIG. 5ashowing the adhesively sealed lid.

FIG. 6 is an exploded view of the micromirror package in FIG. 5a.

FIG. 7 is perspective view of the low-cost micromirror package of thisinvention configured with flexible interconnect cables.

FIG. 8 is a system level diagram of a one-micromirror projection displaysystem incorporating the low-cost micromirror package of this invention.

FIG. 9 is a system level diagram of a 3-micromirror projection displaysystem incorporating three of the low-cost micromirror packages of thisinvention.

DETAILED DESCRIPTION

This invention discloses a low-cost, easily assembled micromirrorpackage. The packages of this invention use fiberglass printed circuitboard substrates, molded plastic parts, and lightweight plastic windowsinstead of ceramic parts and glass windows. The details of the twoembodiments are discussed below.

FIG. 3a shows a first embodiment of the micromirror package 300 of thisinvention. The package is comprised of a printed circuit board (PCB)base (substrate) 30, a molded plastic case 33, a top window-retainingring 37, and a plastic optical window 36, enclosing a micromirror 38.This invention also allows for the attachment of an optical aperture inthe window area of the package, which will prevent stray light fromentering around the edge of the package where it can bounce around andcontaminate the light coming from the micromirror mirrors. The high costceramic used in many conventional packages is replaced by the lower costPCB 30 (example—FR4) and molded plastic 33 assembly. The PCB substrate30 contains circuit traces 31, which are used to bond out themicromirror's 38 leads to the outside by means of either side contacts32 or a grid-pad matrix (not shown) on the bottom of the package 300.Bond wires 39 are shown connecting the micromirror 38 to the PCB traces31. A thermal ground plane is also included on the bottom surface of thePCB 30, which is in effect the bottom of the package 300. The plasticcase 33 is molded around the PCB 30 substrate to form a seal at thebottom of the package. There is an O-ring mating flange 34, shown inFIG. 3b, located on the top of the plastic case 33. The plastic case 33also has snap-pockets where the window 36 mounting-clamp 37 attaches tothe assembly.

FIG. 3b is a sectional view of one side of the package showing the topwindow 36 mounting and clamping mechanism. As shown, the plastic case 33has snap-pockets 40 molded into it to contain the optical plasticwindow-clamping ring 37. An O-ring 35 sits on the O-ring-seal flange 34and then the optical plastic window 36 sits on top of the O-ring.Finally, the mounting clamp 37 is placed over and around the perimeterof the top window 36 and pressed down, compressing the O-ring 35,locking the snap-hooks 41 into the snap-pockets 40 formed in the moldedplastic case 33, sealing the top portion of the package.

FIG. 4 is an exploded view of the micromirror package of the firstembodiment of this invention. The PCB 30 and the molded plastic case 33are mated together in the mold when the plastic case is manufactured,with the PCB 30 becoming the bottom of the package. The micromirror 38is attached to the PCB 30 and bonded out to the circuit interconnecttraces 31, shown in FIG. 3a, using standard semiconductor processes. AnO-ring 35 is then placed on the O-ring-seal flange 34 surface, shown inFIG. 3a, of the package. Finally, the window 36, preferably an opticallyclear plastic window or a glass window, is placed on top of the O-ring35, the mounting clamp 37 is place over and around the edges of thewindow 36, and the snap-hooks of retaining ring 37 are snapped into thesnap-pockets 40 in the plastic case 33, compressing the O-ring, toprovide a lightweight, sealed assembly.

FIG. 5a is a drawing showing a second embodiment of the micromirrorpackage 500 of this invention, which uses an adhesive to attach theoptically clear top window 56. The package is comprised of a printedcircuit board (PCB) base 50, a molded plastic case 53 having anadhesive-seal flange (surface) 54, and an optical window 56, andencloses a micromirror 57. This package is similar to that of the firstembodiment except for the way the optical window 56 is attached. Thebottom of the package is a PCB 50 with circuit traces 51 bringing themicromirror leads out to edge pads 52 or to a bottom grid-pad matrix. Asin the earlier case, the micromirror is attached to the PCB and bondwires 58 are attached between the micromirror input/output pads and thePCB traces 51. A thermal ground plane is also included on the bottomsurface of the PCB 50, which is in effect the bottom of the package 500.The primary difference in this embodiment is that the O-ring matingflange of the earlier embodiment is replaced with an adhesive-sealflange 54, which is an integral part of the molded plastic case 53. Inthis case, the snap pockets in the plastic case are no longer required.This package has all the benefits of the earlier package; low-cost,lightweight, easy assembly, and good reliability.

FIG. 5b is a sectional view of the optical plastic window 56 mountingtechnique for the package in the second embodiment of the invention. Asshown, the plastic case 53 has an adhesive-seal flange 54 built into it.The adhesive 55 is dispensed on top of the seal flange 54 surface andthe plastic window 56 sits on top of the adhesive 55. The adhesive isthen activated and cured to bond the window 56 to the molded case 53 toprovide a completely sealed package.

FIG. 6 is an exploded view of the low-cost micromirror package of thesecond embodiment of this invention. The PCB 50 and the molded plasticcase 53 are mated together in the mold when the plastic case ismanufactured, with the PCB 50 becoming the bottom of the package. Themicromirror 57 is attached to the PCB 50 and bonded out to the circuitinterconnect traces 51 (FIG. 5a) using standard semiconductor processes.An adhesive 55 is then dispensed on to the adhesive-seal flange 54surface of the molded plastic case 53 and the optically clear plasticwindow 56 is placed on top of the adhesive 55. Finally, the adhesive isactivated and cured to properly seal the package.

FIG. 7 shows the micromirror packages of this invention with flexibleinterconnect cables integrally built into the package. Although FIG. 7shows the first embodiment of the invention, either of the two packagesdiscussed above can be configured with flexible interconnect cables. Thepackage shown in FIG. 7, with flex-cable interconnect capability, iscomprised of a printed circuit board base 30 with PCB leads 31, a moldedcase 33 with a snap connected optical plastic window 36, a mountedmicromirror 38, bonding wires 39 connecting the micromirror 38 to thePCB traces 31, and flexible cables 70 (two shown) with lead traces 72and attached connectors 71. This configuration can have up to 4flex-cables. The connectors 71 are used to connect the micromirror(s)into a motherboard or other bus.

FIG. 8 shows a system level block diagram for a single micromirrorprojection display system. The system is comprised of a light source 80,a first condenser lens 81, a motor/color filter wheel assembly 82, asecond condenser lens 83, a low-cost micromirror in the package of thisinvention 84, a fixed or zoom projection lens 85, and a viewing screen86.

Another example of a high-brightness micromirror projection display,which uses three of the low-cost micromirror packages of this invention,is shown in FIG. 9. This system is comprised of a lamp (light source)and reflector assembly 90, a condenser lens 91, a turning mirror 92, atotal internal reflective (TIR) prism 93, three micromirrors (for red,green, and blue light) in the low-cost package of this invention 94,color splitting/color combining prisms 95, a fixed or zoom lens 96, anda viewing screen 97.

While this invention has been described in the context of two preferredembodiments, it will be apparent to those skilled in the art that thepresent invention may be modified in numerous ways and may assumeembodiments other than that specifically set out and described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention that fall within the true spirit andscope of the invention.

What is claimed is:
 1. A MEMS device package comprising: a plastic casewith an O-ring-seal flange; a printed circuit board base in said plasticcase; an O-ring seal on said flange; a window on said O-ring seal; and awindow retaining ring connected to said case and pressing said window tosaid O-ring.
 2. The MEMS package of claim 1, wherein said printedcircuit board is molded into said plastic case.
 3. The MEMS package ofclaim 1, wherein said printed circuit board has a center pad forattaching said MEMS device.
 4. The MEMS package of claim 1, wherein saidprinted circuit board has circuit traces for connecting micromirrorsignals to external circuitry.
 5. The MEMS package of claim 1, whereinsaid plastic case has: corner mounting holes; and built-in retainer ringsnap-slots, said window retaining ring connected to said case via saidsnap-slots.
 6. The MEMS package of claim 1, wherein said printed circuitboard has a thermal ground plane on a bottom surface.
 7. The MEMSpackage of claim 6, wherein said printed circuit board circuit tracesconnect to edge pads around the perimeter of said package.
 8. The MEMSpackage of claim 6, wherein said printed circuit board circuit tracesconnect to a matrix of grid-pads on the bottom surface of said package.9. The MEMS package of claim 6, wherein said printed circuit boardtraces are connected to flex-cables with built-in connectors.
 10. Amethod of packaging a MEMS device, the method comprising the steps of:attaching a MEMS device to a printed circuit board base; bonding pads onsaid MEMS device to pads on said printed circuit board base; placing anO-ring on an O-ring-seal flange of a plastic case; placing a plasticwindow on top of said O-ring; placing a snap-on window retaining ring ontop of and around the perimeter of said window; and compressing saidsnap-on window retaining ring, window, and O-ring until said snap-onretaining ring hooks lock into snap-slots located in said plastic case.11. A MEMS package comprising: a plastic case with an adhesive-sealflange; a printed circuit board base in said plastic case and; a windowbonded to said adhesive-seal flange.
 12. The MEMS package of claim 11,wherein said printed circuit board is molded into said plastic case. 13.The MEMS package of claim 11, wherein said printed circuit board has acenter pad for attaching said micromirror.
 14. The MEMS package of claim11, wherein said printed circuit board has a thermal ground plane on abottom surface.
 15. The MEMS package of claim 11, wherein said printedcircuit board has circuit traces for connecting micromirror signals toexternal circuitry.
 16. The MEMS package of claim 15, wherein saidprinted circuit board circuit traces connect to edge pads around theperimeter of said package.
 17. The MEMS package of claim 15, whereinsaid printed circuit board circuit traces connect to a matrix ofgrid-pads on the bottom surface of said package.
 18. The MEMS package ofclaim 15, wherein said printed circuit board traces are connected toflex-cables with built-in connectors.
 19. A method of packaging amicromirror, said method comprising the steps of: attaching amicromirror to a printed circuit board base; bonding micromirror pads toprinted circuit board base pads; placing said printed circuit board in aplastic case; dispensing adhesive onto an adhesive-seal flange on saidplastic case; placing a window on top of said adhesive; and curing saidadhesive.